Information display apparatus

ABSTRACT

The information display apparatus configured to display video information of a virtual image on a reflecting surface of conveyance includes: a display configured to display the video information; and a virtual image optical system configured to display a virtual image at a front of the conveyance by reflecting light emitted from the display by means of the reflecting surface. The virtual image optical system includes a concave mirror and an optical element. The optical element is arranged between the display and the concave mirror, and is configured to correct distortion of the virtual image obtained so as to correspond to a viewpoint position of a driver on a basis of a shape of the concave mirror and a shape of the optical element. The information display apparatus further includes a virtual image double image conversion reducer configured to reduce double image conversion of the virtual image.

TECHNICAL FIELD

The present invention relates to an information display apparatus thatprojects that projects an image on a windshield or a combiner of avehicle, an electric train, an airplane or the like (hereinafter,generally referred to as “conveyance”). The present invention alsorelates to a projection optical system in which an image is observedthrough the windshield as a virtual image and an information displayapparatus using the same.

BACKGROUND ART

A so-called head up display (HUD: Head Up Display) apparatus has alreadybeen known by Patent Document 1 below. The head up display apparatusprojects video light onto a windshield or a combiner of a vehicle toform a virtual image, thereby displaying traffic information such asroute information or traffic jam information and vehicle informationsuch as a remaining amount of fuel or cooling water temperature thereon.

In this type of information display apparatus, it is desired that aregion that a driver can view a virtual image is enlarged. However, itis an important performance factor that a virtual image has highresolution and high visibility.

A head up display apparatus necessarily requires a windshield orcombiner as a final reflecting surface by which a virtual image isprovided to a driver. The inventors of the present application noticedthat improvement of a double image of a virtual image, which isgenerated by double reflection that occurs on the windshield or combineras the final reflecting surface, was important in order to obtain goodresolution performance with high visibility.

On the other hand, for example, an apparatus whose body including acombiner is attached to the vicinity of a ceiling (or a sun visor) of avehicle as also disclosed in Non-Patent Document 1 below has alreadybeen proposed.

RELATED ART DOCUMENTS Patent Documents

Patent document 1: Japanese Unexamined Patent Application PublicationNo. 2015-194707

Non-Patent Documents

Non-Patent document 1: PIONEER R&D (Vol. 22, 2013)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As shown in FIG. 33, a principle of generation of a virtual image by aconcave mirror to realize a head up display apparatus according to aconventional technique arranges an object point AB at an inner side of afocal point F (focal point distance f) with respect to a point O on anoptical axis of a concave mirror 1′, thereby allowing the virtual imageby the concave mirror 1′ to be obtained. In FIG. 33, for convenience ofexplanation, the concave mirror 1′ is regarded as a convex lens with thesame positive refractive power, and a relationship among an objectpoint, the convex lens (described by the concave mirror in FIG. 33 forconvenience of explanation), and the virtual image to be generated isshown.

In the conventional technique, in order to enlarge a size of the virtualimage to be generated on the concave mirror 1′, the object point AB maybe caused to approach the focal point F and the concave mirror may beenlarged with respect to an object size AB. However, in order to obtaindesired magnification, a radius of curvature of the concave mirrorbecomes smaller. Thus, it is difficult to establish both of these. As aresult, a mirror size becomes smaller, and this results in a state whereonly a virtual image whose magnification power is effectively large buta viewable range is small can be obtained. For this reason, in order tosatisfy (1) a desired size of a virtual image, and (2) necessarymagnification of the virtual image M=b/a at the same time, it isnecessary that a dimension of the concave mirror is fitted to a viewingrange and magnification of the virtual image is determined in view of avideo display apparatus.

For this reason, in the conventional technique, in order to obtain alarge virtual image in a desired view range, for example, as illustratedin FIG. 33, a distance from the concave mirror 1′ to a virtual imageneeded to be enlarged. Namely, there was necessary that a distancebetween the windshield or combiner (not illustrated in the drawings) asthe final reflecting surface and the concave mirror is to be enlargedand a size of the concave mirror is to be enlarged at the same time.However, in the conventional technique, a double image of a virtualimage generated by double reflection that occurs on a windshield orcombiner has never been considered.

Further, for example, in the example of the head up display apparatusdisclosed in Patent Document 1 described above, which is theconventional technique, the head up display apparatus includes a deviceconfigured to display an image and a projection optical systemconfigured to project the image displayed by the display device. Theprojection optical system includes a first mirror and a second mirror onan optical path of a viewer from the display device. Miniaturization isrealized by satisfying predetermined conditions for a relationship amongan incident angle of the first mirror in a long axis direction of theimage, an incident angle of the first mirror in a short axis directionof the image, an interval between an image display surface of thedisplay device and the first mirror, and a width of a virtual imageviewed and recognized by the viewer in a horizontal direction. However,means for reducing the double image described above, which is generatedby reflection of both surfaces of a windshield (two surfaces at adriver's side and an outside) is not described in Patent document 1.

On the other hand, in an apparatus whose body is attached to thevicinity of a ceiling (or a sun visor) of a vehicle as disclosed inNon-Patent document 1, generation of a double image is reduced byforming antireflection coating on a reflecting surface to which a driverdoes not face. However, problems on safety still remains such as apossibility that a driver may get an injury in a case where a collisionaccident occurs and an HUD apparatus thereby comes off.

For this reason, it is thought that a system in which a windshield isused as a reflecting surface disclosed in Patent document 1 describedabove becomes mainstream in the future. Therefore, technical means forreducing reflection of video light generated by virtual images that arereflected by both surfaces of a windshield by means of contrivance of aprojection optical system has been invented.

The present invention including technical means for reducing conversionfrom a virtual image to a double image (hereinafter, referred to as“double image conversion”), which may be generated in a case where awindshield is used as a reflecting surface (will be described below indetail), by contrivance of an optical system is proposed. In addition,it is an object of the present invention to provide an informationdisplay apparatus capable of forming a virtual image with highvisibility in which distortion and aberration of the virtual imageviewed and recognized by a driver are reduced up to a level practicallyhaving no problem.

Means for Solving the Problem

One example of the present invention that is made in order to achievethe object described above is an information display apparatusconfigured to display video information of a virtual image on areflecting surface of conveyance, the information display apparatusincluding: a display configured to display the video information; and avirtual image optical system configured to display a plurality ofvirtual images at a front of the conveyance by reflecting light emittedfrom the display by means of the reflecting surface. The virtual imageoptical system includes a concave mirror and an optical element. Theoptical element optimizes a shape and a position of the optical elementin accordance with a video light flux in which each of the virtualimages separated between the display and the concave mirror isestablished so that a virtual image such as risk information to besuperimposed on a distant view, for example, is established at an upperportion of the reflecting surface and a virtual image such as speedinformation or an arrow indicating a traveling direction of navigationto be superimposed on a near view or a hood of a vehicle is establishedfrom the upper portion of the reflecting surface toward a lower portionthereof. By providing means for reducing double image conversion of avirtual image occurring due to reflection of the video light flux by afront surface and a back surface of the reflecting surface, theplurality of virtual images is formed at a plurality of positionscorresponding to a viewpoint position of a driver while reducing a sizethereof.

Thus, according to the present invention, an information displayapparatus that controls a divergent angle of light incident on anoptical system of video light from a video display apparatus so as toreduce a double image generated in the virtual image described above andobtain video whose visibility is improved is realized.

Effects of the Invention

According to the present invention, it becomes possible to provide aninformation display apparatus that forms a virtual image whosevisibility is improved while realizing miniaturization of the apparatusby controlling a divergent angle of a light source light flux incidenton a virtual image optical system in order to correct distortion and/oraberration of the virtual image observed by a driver and reduce a doubleimage generated in the virtual image.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram illustrating a peripheralequipment configuration of an information display apparatus according toan embodiment;

FIG. 2 is a top view of a vehicle on which the information displayapparatus is mounted;

FIG. 3 is a view for explaining a difference of a radius of curvature ofa windshield;

FIG. 4 is a schematic configuration diagram illustrating the informationdisplay apparatus, the windshield, and a viewpoint position of a driver;

FIG. 5 is a schematic configuration diagram illustrating one embodimentof a virtual image optical system in the information display apparatus;

FIG. 6 is a ray diagram of the whole virtual image optical system in theinformation display apparatus according to the embodiment;

FIG. 7 is a ray diagram of a part of the virtual image optical system inthe information display apparatus according to the embodiment;

FIG. 8 is a schematic explanatory diagram for explaining a principle ofthe virtual image optical system;

FIG. 9 is a schematic view for explaining a principle of generation of adouble image;

FIG. 10 is a schematic explanatory diagram for explaining a principle ofthe present invention;

FIG. 11 is a view illustrating resolution performance evaluation pointsin EyeBox according to the embodiment;

FIG. 12 is a spot diagram illustrating a reverse tracking result by agreen ray indicating a result of resolution performance evaluationaccording to the embodiment (an image forming state at a video source isevaluated by flying a ray from a virtual image);

FIG. 13 is a view illustrating distortion performance according to theembodiment when viewed from a center of the EyeBox;

FIG. 14 is lens data according to the embodiment;

FIG. 15 is lens data according to the embodiment;

FIG. 16 is a configuration diagram illustrating arrangement of the videodisplay apparatus and a light source apparatus;

FIG. 17 is a schematic configuration diagram illustrating aconfiguration of the light source apparatus;

FIG. 18 is a schematic view for explaining an emission status of alightflux from the video display apparatus and the light source apparatus;

FIG. 19 is a characteristic diagram for explaining emission lightdistribution of the light flux from the light source apparatus;

FIG. 20 is a schematic configuration diagram illustrating a shape of alight guide element in the light source apparatus;

FIG. 21 is a schematic configuration diagram illustrating across-sectional shape of the light guide element in the light sourceapparatus;

FIG. 22 is a conceptual diagram illustrating a method of characteristicevaluation for a liquid crystal panel;

FIG. 23 is a characteristic diagram illustrating a transmittancecharacteristic of a screen in a horizontal direction of the liquidcrystal panel;

FIG. 24 is a characteristic diagram illustrating an angularcharacteristic of brightness of the screen in the horizontal directionin a case where white is displayed on the liquid crystal panel;

FIG. 25 is a characteristic diagram illustrating an angularcharacteristic of backlight brightness in the horizontal direction ofthe liquid crystal panel;

FIG. 26 is a characteristic diagram illustrating an angularcharacteristic of backlight brightness in the vertical direction of theliquid crystal panel;

FIG. 27 is a characteristic diagram illustrating an angularcharacteristic of contrast in the horizontal direction of the liquidcrystal panel;

FIG. 28 is a characteristic diagram illustrating a transmittancecharacteristic of the liquid crystal panel in the vertical direction;

FIG. 29 is a characteristic diagram illustrating an angularcharacteristic of brightness of the liquid crystal panel in the verticaldirection;

FIG. 30 is a characteristic diagram illustrating an angularcharacteristic of black display brightness in the horizontal directionof the liquid crystal panel;

FIG. 31 is a characteristic diagram illustrating an angularcharacteristic of the contrast in the vertical direction of the liquidcrystal panel;

FIG. 32 is a characteristic diagram illustrating an angularcharacteristic of the black display brightness in the vertical directionof the liquid crystal panel;

FIG. 33 is a schematic view for explaining a principle of a virtualimage optical system according to a conventional technique;

FIG. 34 is a schematic view for explaining a change in a reflectionfactor of glass based on an incident angle by S-polarized light andP-polarized light;

FIG. 35 is a schematic configuration diagram for explaining a concretemethod of reducing Petzval sum of the virtual image optical system; and

FIG. 36 is a schematic configuration diagram for explaining a concretemethod of reducing Petzval sum of the virtual image optical system.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the drawings and the like. Note that thepresent invention is not limited to the following explanation, andvarious changes and modifications can be made by a person havingordinary skill in the art within a scope of technical ideas disclosed inthe present specification. Further, in all of the drawings forexplaining the present invention, the same reference numeral may beattached to a component having the same function, and repeatedexplanation may be omitted.

<Embodiments of Information Display Apparatus>

FIG. 1 is a block diagram and a schematic configuration diagramillustrating a peripheral equipment configuration of an informationdisplay apparatus according to one embodiment of the present invention.Here, an information display apparatus 100 configured to project animage onto a windshield of a vehicle will be described particularly asone example thereof.

This information display apparatus 100 is an apparatus (that is, aso-called HUD (Head Up Display)) configured to display, as a virtualimage VI (Virtual Image), various kinds of information that arereflected by a projected member 6 (in the present embodiment, an innersurface of the windshield) in order to form a virtual image V1 at afront side of an own vehicle along a line of sight 8 of a driver. Inthis regard, the projected member 6 may be a member on which informationis projected, and it may be not only the windshield described above, butalso a combiner. Namely, in the information display apparatus 100according to the present embodiment, the projected member 6 may be amember that allows the driver to view (and recognize) a virtual imagethat is formed at the front side of the own vehicle along the line ofsight 8 the driver. As a matter of course, vehicle information, andinformation on the foreground photographed by cameras (not shown in thedrawings), such as a monitoring camera or an around viewer, arecontained as the information to be displayed as the virtual image, forexample.

Further, the information display apparatus 100 includes a video displayapparatus 4, a concave mirror 1, and a group of lenses 2 for correctionprovided between the video display apparatus 4 and the concave mirror 1.The video display apparatus 4 is configured to project video light todisplay information. The group of lenses 2 is configured to correctdistortion and/or aberration that occurs when a virtual image is formedby a concave mirror 1 from video displayed by the video displayapparatus 4.

The information display apparatus 100 also includes a controller 40configured to control the video display apparatus 4 described above anda backlight 5. Note that optical components including the video displayapparatus 4 and the backlight 5 described above is a virtual imageoptical system, which will be described later, and includes the mirrorconcave 1, which reflects light. Further, the light reflected by theseoptical components is reflected by the projected member 6 to go towardthe line of sight 8 of the driver (EyeBox: which will be describedlater).

As the video display apparatus 4 described above, for example, there area light emitting VFD (Vacuum Fluorescent Display) and the like inaddition to an LCD (Liquid Crystal Display) having a backlight.

On the other hand, in place of the video display apparatus 4 describedabove, video may be displayed on a screen by a projection apparatus, avirtual image may be formed therefrom by the concave mirror 1 describedabove and reflected by a windshield 6 that is the projected member todirect to the line of sight 8 of the driver.

As such a screen, for example, the screen may be configured by amicrolens array in which microlenses are arranged thereon in atwo-dimensional manner.

Here, in order to reduce distortion of the virtual image, a shape of theconcave mirror 1 may have a shape in which a radius of curvature at anupper portion shown in FIG. 1 (that is, an area where a ray is reflectedat a lower part of the windshield 6 at which a distance from a viewpointof the driver is relatively short) so that magnification power thereofbecomes larger is relatively small, and a radius of curvature at a lowerportion thereof (that is, an area where a ray is reflected at an upperpart of the windshield 6 at which a distance from the viewpoint of thedriver is relatively long) so that magnification power thereof becomessmaller is relatively large. Further, a difference of virtual imagemagnifications described above is corrected by inclining the videodisplay apparatus 4 with respect to an optical axis of the concavemirror to reduce distortion itself that may occur, whereby goodcorrection can further be realized.

On the other hand, as shown in FIG. 2 and FIG. 3, with respect to thewindshield 6 of the vehicle, a radius of curvature Rv in a verticaldirection of a body thereof is different from a radius of curvature Rhin a horizontal direction, and they generally have a relationship ofRh>Rv. For this reason, when the windshield 6 is grasped as a reflectingsurface, it becomes a toroidal surface of the concave mirror. For thisreason, in the information display apparatus 100 of the presentembodiment, the shape of the concave mirror 1 may have different averageradii of curvature between the horizontal direction and the verticaldirection so as to correct the virtual image magnification by a shape ofthe windshield 6, that is, so as to correct a difference between theradii of curvature in the vertical direction and the horizontaldirection of the windshield 6. In this case, in a case where the shapeof the concave mirror 1 is a spherical or aspherical shape symmetricabout the optical axis (that is, a shape expressed by Formula 2, whichwill be described later), it becomes a function of a distance r from theoptical axis, a horizontal cross-sectional shape and a verticalcross-sectional shape of separated places cannot be controlledindividually. Therefore, it is preferable to correct the difference as afunction of a coordinate (x, y) on a surface from the optical axis of amirror surface as a free-form surface expressed by Formula 1 (will bedescribed later).

The explanation returns to FIG. 1 again. A lens element 2 is furtherarranged between the video display apparatus 4 and the concave mirror 1as a transmissive optical component, for example. By controlling anemission direction of the ray to the concave mirror 1, aberrationcorrection of the virtual image including astigmatism that occurs due tothe difference between the radius of curvature in the horizontaldirection and the radius of curvature in the vertical direction of thewindshield 6 described above is realized at the same time as correctionof distortion aberration is carried out in accordance with the shape ofthe concave mirror 1.

Further, in order to further heighten aberration correction capability,an optical element 2 as described above may be constituted by aplurality of lenses. Alternatively, by arranging a curved mirror inplace of the lens element and controlling an incident position of theray to the concave mirror 1 at the same time of folding of the opticalpath, it is possible to reduce distortion aberration. As describedabove, it goes without saying that it does not depart from technicalideas or a range of the present invention even though an optical elementoptimally designed to improve the aberration correction capability isfurther provided between the concave mirror 1 and the video displayapparatus 4. Moreover, by changing a thickness of the optical element 2described above in an optical axis direction, an optical distancebetween the concave mirror 1 and the video display apparatus 4 can bechanged in addition to true aberration correction, whereby it ispossible to continuously change a display position of the virtual imagefrom a distant place to an adjacent position.

Further, by arranging the video display apparatus 4 so as to incline thesame with respect to a normal line of the optical axis of the concavemirror 1, the difference of magnifications of the virtual image in thevertical direction may be corrected.

On the other hand, as a factor to deteriorate image quality of theinformation display apparatus 100, there is known a fact that a videolight ray emitted from the video display apparatus 4 toward the concavemirror 1 is reflected by a surface of the optical element 2 arranged inthe middle to return to the video display apparatus 4, and is thenreflected again to be superimposed on true video light, whereby theimage quality thereof is deteriorated. For this reason, in the presentembodiment, it is preferable that the information display apparatus 100is designed so that not only antireflection coating is formed on thesurface of the optical element 2 to suppress the reflection, but also alens surface shape of any one or both of an incident surface and anemission surface of the optical element 2 for the video light is causedto have restriction for the surface shape to become a shape so that thereflected light described above does not focus on apart of the videodisplay apparatus 4 (for example, a shape in which a concave surfacefaces the video display apparatus 4).

Next, in a case where, as the video display apparatus 4, a secondpolarizing plate is arranged so as to be separated from a liquid crystalpanel in addition to a first polarizing plate arranges in proximity tothe liquid crystal panel in order to absorb the reflected light from theoptical element 2 described above, it is possible to reducedeterioration in image quality. Further, the backlight 5 of the liquidcrystal panel is controlled so that an incident direction of lightincident on the liquid crystal panel 4 efficiently enters an incidenteye of the concave mirror 1. At this time, by making a divergent angleof a light flux incident on the liquid crystal panel smaller, it becomespossible to not only effectively direct video light toward eye points ofthe driver, but also obtain video with high contrast and good visibilityas illustrated in FIG. 27 and FIG. 31. Contrast performance in thehorizontal direction with respect to the divergent angle of the video isremarkable. When a visual angle is within ±20°, an excellent propertycan be obtained. In order to further improve the contrast performance, alight flux whose visual angle is within ±10° may be used.

On the other hand, it is preferable that a solid light source with longproduct lifetime is adopted as the light source. Moreover, it ispreferable that polarization conversion is carried out by using a PBS(Polarizing Beam Splitter) in which optical means for reducing adivergent angle of light as LED (Light Emitting Diode) whose change inlight output with respect to variation in ambient temperature is smallis provided.

Polarizing plates are respectively arranged at the backlight 5 side(light incident surface) and the optical element 2 side (light emissionsurface) of the liquid crystal panel. This makes it possible to heightena contrast ratio of the video light. In a case where iodine-based one inwhich the degree of polarization is high is adopted for the polarizingplate provided at the backlight 5 side (light incident surface), a highcontrast ratio can be obtained. On the other hand, by using a dye-basedpolarizing plate for one provided at the optical element 2 side (lightemission surface), it becomes possible to obtain high reliability evenin a case where outside light enters or ambient temperature is high.

In a case where the liquid crystal panel is used as the video displayapparatus 4, in particular, in a case where the driver wears polarizedsunglasses in such a situation, a specific polarized wave is blocked orshielded, whereby a defect in which video cannot be viewed occurs. Inorder to prevent this situation, it is preferable that a λ/4 plate isarranged at a side of the optical element of the polarizing plate thatis arranged at the optical element 2 side of the liquid crystal panel,whereby the video light uniformed to a specific polarization directionis converted into circular polarized light.

The controller 40 obtains, from such a navigation system 61, variouskinds of information such as a speed limit and the number of lanes of aroad corresponding to a current position at which the own vehicle istravelling, and a scheduled movement route of the own vehicle set to thenavigation system 61 as foreground information (that is, information tobe displayed at a front of the own vehicle by the virtual imagedescribed above).

A driving support ECU 62 is a controller that realizes driving supportcontrol by controlling a drive system and a control system in accordancewith an obstacle detected as a result of monitoring by a circumferencemonitoring device 63. The driving support control includes well-knowntechnologies such as cruise control, adaptive cruise control, pre-crushsafety, lane keeping assist, for example.

The circumference monitoring device 63 is a device for monitoring astatus of a circumference of the own vehicle. As one example, there area camera that detects an object existing on the circumference of the ownvehicle on the basis of an image obtained by photographing thecircumference of the own vehicle, an exploratory device that detects anobject existing around the own vehicle on the basis of a result obtainedby transmitting and receiving an exploratory wave, and the like.

The controller 40 obtains such information from the driving support ECU62 (for example, a distance to a preceding vehicle and a direction ofthe preceding vehicle, a position at which an obstacle or a traffic signexists, and the like) as foreground information. Moreover, an ignition(IG) signal and own vehicle state information are inputted to thecontroller 40. The own vehicle state information among these kinds ofinformation is information obtained as the vehicle information. Forexample, the own vehicle state information contains warning informationindicating to become an abnormal state defined in advance, such asresidual quantity of fuel for an internal-combustion engine ortemperature of cooling water. Further, the own vehicle state informationalso contains an operational result of a direction indicator, travellingspeed of the own vehicle, and shift position information. The controller40 that has been mentioned above is activated when the ignition signalis inputted thereto. As the above, the whole system of the informationdisplay apparatus according to the present embodiment has beendescribed.

<First Embodiment of Virtual Image Optical System>

Next, further details of a virtual image optical system according to thepresent embodiment and the video display apparatus will be describedbelow.

As have already been mentioned above, FIG. 2 is a top view of thevehicle on which the information display apparatus 100 according to thepresent embodiment is mounted. A windshield exists as the projectedmember 6 in front of a driver's seat of a vehicle body 101. Note that aninclination angle of this windshield with respect to the vehicle body isdifferent depending upon a type of the vehicle. Moreover, the inventorsof the present application researched this radius of curvature in orderto realize an optimum virtual image optical system. As a result, asshown in FIG. 3, the inventors found that in the windshield, the radiusof curvature Rh in the horizontal direction parallel to a contactsurface of the vehicle is different from the radius of curvature Rv inthe vertical direction orthogonal with respect to a horizontal axis, andthere is generally a relationship below between Rh and Rv.

Rh>Rv

Further, it was also found that most of vehicles has this differencebetween the radii of curvature, that is, the Rh with respect to Rv in arange from 1.5 times to 2.5 times.

Next, the inventors also researched commercial products with respect tothe inclination angle of the windshield. As a result, although it isdifferent depending upon a vehicle body type, it was 20° to 30° in alight motor vehicle or a minivan type, 30° to 40° in a sedan type, and40° or more in a sports type. Thus, in the present embodiment, adifference between the radius of curvature Rh in the horizontaldirection of the windshield parallel to the contact surface of thevehicle and the radius of curvature Rv in the vertical directionorthogonal to the horizontal axis and the inclination angle of thewindshield were considered to design the virtual image optical system.

More specifically, since the horizontal radius of curvature Rh and thevertical radius of curvature Rv of the windshield that is the projectedmember are greatly different from each other, good aberration correctionwas realized by providing the optical element 2 in the virtual imageoptical system. The optical element 2 is axially asymmetric with thehorizontal axis of the windshield with respect to the optical axis (Zaxis) and the axis vertical to this axis.

Next, the inventors carried out examination for miniaturization of theinformation display apparatus 100. As a condition of the examination,horizontal: 7° and vertical: 2.6° of FOV were set, and a virtual imagedistance is further set to 2 m, whereby the examination was carried out.At first of the examination, the concave mirror 1 to generate a virtualimage (which is simply displayed as a plane mirror in FIG. 5 below), thevideo display apparatus 4, and the backlight 5 were prepared as a basicconfiguration, and one optical path folding mirror was arranged betweenthe video display apparatus 4 and the concave mirror 1. Simulation wascarried out by using, as parameters, arrangement of the respectivemembers and a distance from the video display apparatus 4 to the concavemirror 1 so that volume of the information display apparatus 100 becomesthe minimum.

As a result, the volume when they were arranged so that video light fromthe video display apparatus 4 does not interfere with any of the membersbecame 3.6 liters. Then, examination about a direct method in which theoptical path folding mirror is removed was carried out for furtherminiaturization.

A configuration of the virtual image optical system according to thepresent invention will be described with reference to FIG. 4. FIG. 4 isthe configuration diagram illustrating a basic configuration to conducta study for miniaturization in the virtual image optical system shown inFIG. 1 according to the present embodiment. In order to simplifydescription thereof, an optical element for correcting aberration anddistortion aberration is omitted, and across-sectional shape in avertical direction is shown as well as the windshield 6 shown in FIG. 1.The liquid crystal panel is assumed as the video display apparatus 4,and the video display apparatus 4 is arranged at a position at which thevirtual image of displayed video is obtained by the concave mirror 1 asa basic configuration in which the backlight 5 is arranged.

At this time, as shown in FIG. 5, each of video light R2 generated fromvideo at a center, video light R1 generated from video at an upper end,and video light R3 generated at a lower end of the screen of the videodisplay apparatus 4 is arranged so that the light is not interfered andblocked out by the video display apparatus 4 when they are reflected bythe concave mirror 1, which becomes restriction in design.

The inventors set horizontal: 7° and vertical: 2.6° of the FOV at thesame time in view of the design restriction described above, and furtherset a virtual image distance to 2 m, thereby determining volume of theinformation display apparatus 100 by using an interval Z between theconcave mirror 1 and the video display apparatus 4 (the liquid crystalpanel and the backlight 5) as a parameter. In a case where a distance Zis 100 mm, a vertical dimension of the concave mirror 1 can be made thesmallest. In a case where the distance Z is set to 75 mm, an angle α₂between a horizontal plane and the concave mirror 1 becomes larger, andthe vertical dimension of the concave mirror 1 also becomes larger. Whenthe distance Z is further reduced to be equal to or less than 50 mm, anangle α₃ between the horizontal plane and the concave mirror 1 furtherbecomes larger, and the vertical dimension of the concave mirror 1 alsofurther becomes larger.

As described above, a relationship between a set height and a set depthof the video display apparatus 4 was simulated by using the distance Zas a parameter. It is possible to reduce the set depth by making thedistance Z smaller. However, the set height becomes higher. Similarly,in a relationship between the distance Z and set volume L, a change inthe set volume (containing volume of an LCD driving circuit, a lightsource driving circuit, and a backlight unit) changed as a boundary whenthe distance Z is 60 mm compared with volume of a space from the videodisplay apparatus 4 to the concave mirror (optical path volume anddisplay).

From the above, in order to miniaturize the information displayapparatus 100, it was found that it is necessary to realize the virtualimage optical system in which the distance Z to directly enlarge videodisplayed by the video display apparatus 4 with the concave mirror isshort, and that it is necessary for the center of a video display unitof the video display apparatus 4 in the vertical direction of the screento be arranged at a lower side than the center of the concave mirror 1.

On the other hand, in this arrangement, a distance (corresponding to theray R1) between the video display apparatus 4 and the upper end of theconcave mirror 1 becomes long, while a distance (corresponding to theray R3) between the video display apparatus 4 and the lower end of theconcave mirror 1 becomes short. Thus, the video display apparatus 4 maybe arranged so that the distance between the video display apparatus 4and the concave mirror 1 becomes even as much as possible.

In the virtual image optical system according to the present embodiment(see FIG. 6 and FIG. 7), distortion correction of the virtual imagebetween the video display apparatus 4 and the concave mirror 1 andaberration correction by the optical element that corrects aberrationoccurring by the virtual image are carried out. This will be describedlater with reference to FIG. 8. Namely, by arranging the video displayapparatus 4 (object point) inside a focal point F (focal point distancef) with respect to a point O on the optical axis of the concave mirror1, it is possible to obtain the virtual image by the concave mirror 1.In FIG. 8, for convenience of explanation, the concave mirror 1 isregarded as a convex lens with the same positive refractive power, and arelationship among the object point, the convex lens (described by theconcave mirror in FIG. 8 for convenience of explanation), and thevirtual image to be generated is shown.

In the present embodiment, the optical element 2 is arranged in order toreduce distortion and aberration that occur on the concave mirror 1.This optical element may be configured by a transmissive optical lens ora concave mirror. However, a direction of the light flux incident on theconcave mirror (an angle and a position thereof) is controlled so that:

-   (1) in a case where the video light from the video display apparatus    4 is made incident on the reflecting surface as a telecentric light    flux, refractive power of the lens or the concave mirror 1    substantially becomes zero;-   (2) in a case where the video light from the video display apparatus    4 is diverged and made incident on the optical element, the optical    element has positive refractive power; and-   (3) in a case where the video light from the video display apparatus    4 is focused and made incident on the optical element, the optical    element has negative refractive power.    Hereby, the distortion aberration of the generated virtual image is    corrected. Moreover, in case of the transmissive optical lens,    aberration regarding image forming performance, which occurring in    the virtual image, is corrected by interaction between an incident    surface at the video display apparatus 4 side and an emission    surface at the concave mirror 1 side. The case of one optical    element has been described in the present embodiment. However, this    may be configured by a plurality of transmissive optical lenses.    Alternatively, this may be configured by a combination of a    reflective optical lens (or mirror) and a transmissive optical lens.

At this time, as described above, in a size of the virtual image viewedand recognized by the driver, each of a distance a between the videodisplay apparatus 4 and the concave mirror 1 and a distance b betweenthe concave mirror 1 and the virtual image, which are generated due toinclination of the windshield, is different between the upper end andthe lower end of the virtual image. For that reason, a double image of avirtual image is generated in a case where the windshield or combiner isused as the reflecting surface. Thus, in the present embodiment, atechnique to reduce double image conversion of the virtual image thatoccurs in such a case by contrivance of an optical system has beendeveloped. Its details will be described below.

<Mechanism of Occurrence of Double Image Conversion of Virtual Image>

As a result of various kinds of investigation, the inventors developed atechnique to reduce double image conversion of a virtual image, whichwill be described below in detail, on the basis of knowledge mentionedbelow.

As illustrated in FIG. 9, with respect to a virtual image that isreflected and then viewed and recognized by the driver at an upperportion of windshield 6, a ray that generates the virtual imagedescribed above is obliquely made incident on the windshield 6 due toinclination of the windshield 6. Thus, when a thickness of thewindshield 6 is set to t, a reflection position a of regular lightreflected by a reflecting surface near the driver (hereinafter, referredto as a “reflecting surface 1 (or first reflecting surface)”) and areflection position b of back surface reflected light reflected by areflecting surface far from the driver (hereinafter, referred to as a“reflecting surface 2 (or second reflecting surface)”) are shiftedupward to each other by a distance L in the vertical direction, wherebytwo virtual images are formed.

A reflection factor of light incident on the windshield from the air is4%, and a reflection factor by an interface between the windshield andthe air is 4%. They are the same. Ina regular virtual image by theregular light and a second virtual image by the back surface reflectedlight, brightness by the regular virtual image is nearly equal tobrightness by the second virtual image by the back surface reflectedlight. For this reason, reduction of brightness of the virtual image bythe back surface reflected light becomes absolutely necessary in orderto obtain good resolution performance of video by the virtual image.

On the other hand, as well as the reflection at the upper portion asillustrated in FIG. 9, in the virtual image reflected at the lowerportion of the windshield 6 and then viewed and recognized by thedriver, the ray that generates the virtual image described above isobliquely made incident on the windshield 6 due to inclination ofwindshield 6. The back surface reflected light of the other is shiftedupward with respect to regular reflected light, whereby two virtualimages are formed.

Moreover, in the horizontal direction of the screen, the back surfacereflected light is shifted in a direction away from a point at which theoptical axis of the concave mirror crosses the windshield with respectto the regular reflected light, whereby two virtual images are formed.

In a case where a refractive index of the windshield is set to 1.5 asdescribed above, a relationship between an incident angle of a videolight ray to the windshield and a reflection factor is illustrated inFIG. 34. In case of vertical incidence, both reflection factors withrespect to S-polarized light and P-polarized light are about 4%.However, when the incident angle exceeds 25°, the reflection factor withrespect to the S-polarized wave becomes larger.

For this reason, in a case where an LCD is used as the video displayapparatus, the reflection factor of the windshield varies depending uponwhich polarized light is used as video output light. Thus, there is apossibility that brightness of the virtual image, which the driver canview and recognize, may vary depending upon the incident angle to thereflecting surface.

Moreover, when a distance between the windshield and the concave mirroris not changed and a region that the driver can view and recognize thevirtual image is enlarged, an angle of the video light ray incident onthe windshield becomes larger, and double images are generated in thevertical direction and the horizontal direction of the screen. Thisinterferes with a focus feeling of the virtual image.

For this reason, the inventors found that it is further better to reducedistortion aberration occurring due to this by inclining the videodisplay apparatus 4 with respect to the optical axis of the concavemirror 1 as shown in FIG. 9 to substantially match image magnificationM′=b′/a′ of an upper end portion of the virtual image with imagemagnification M=b/a of a lower end portion of the virtual image.

Moreover, by setting an average radius of curvature of a cross-sectionalshape of the optical element 2 in a vertical direction and an averageradius of curvature of a cross-sectional shape in a horizontal directionto different values, distortion aberration occurring by an optical pathdifference that occurs due to a difference between the radius ofcurvature Rv in the vertical direction and the radius of curvature Rh inthe horizontal direction of the windshield described above andaberration that deteriorates the image forming performance of thevirtual image are corrected.

As mentioned above, in the information display apparatus 100 thatobtains a virtual image by directly reflecting video light to thewindshield 6, correction of aberration occurring by the optical pathdifference that occurs due to the difference between the radius ofcurvature Rv in the vertical direction and the radius of curvature Rh inthe horizontal direction of the windshield 6 becomes the most importantfor securement of the image forming performance of the virtual image.

For this reason, the inventors reduced deterioration of the imageforming performance of the virtual image due to the difference betweenthe radii of curvature of the windshield described above by using afree-form surface shape (see Formula 1 below) capable of defining ashape of a surface as a function of an absolute coordinate (x, y) fromthe optical axis against an aspherical shape (see Formula 2 below),which has been used in a conventional optical design, to define a shapeof a lens surface or mirror surface as a function of the distance r fromthe optical axis.

$\begin{matrix}{{Z = {\frac{c \cdot ( {x^{2} + y^{2}} )}{1 + \sqrt{1 - {( {1 + K} ){c^{2} \cdot ( {x^{2} + y^{2}} )}}}} + {\sum{\sum( {{{Cj}( {m,n} )} \times x^{m} \times y^{n}} )}}}}{j = {{\lbrack {( {m + n} )^{2} + m + {3n}} \rbrack/2} + 1}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

Note that the aspherical shape to define the shape of the lens surfaceor mirror surface is expressed as a function of the distance r from theoptical axis like Formula 2 below.

$\begin{matrix}{Z = {\frac{c \cdot h^{2}}{1 + \sqrt{1 - {( {1 + K} ){c^{2} \cdot h^{2}}}}} + {A \times h^{4}} + {B \times h^{6}} + {C \times h^{8}} + {D \times h^{10}} + {E \times h^{12}} + {F \times h^{14}} + {G \times h^{16}} + {H \times h^{18}} + {J \times h^{20}}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

A refractive index of a windshield for a vehicle is generally 1.5(n=1.5), and a reflection factor per one surface thereof is 5%. Asdescribed above, the information display apparatus 100 causes a virtualimage to be reflected by the windshield to form video in the EyeBox ofthe driver. For this reason, as illustrated in FIG. 9, a video light rayis separated into regular reflected light reflected by the reflectingsurface 1 positioned at an internal side of the vehicle of windshield 6and back surface reflected light reflected by the reflecting surface 2that is in contact with the outside air. It is recognized as a doubleimage by eyes of the driver. A direction in which this double image isgenerated is different between the vertical direction and the horizontaldirection of the windshield 6. In a case where the information displayapparatus 100 is arranged under the windshield 6, a double image that isgenerated by the back surface reflected light is generated at an upperportion of video by the regular reflected light.

Similarly, even in a case where the information display apparatus 100 isarranged above the windshield, a double image generated by the backsurface reflected light is generated at the upper portion of the videoby the regular reflected light.

On the other hand, a double image in the horizontal direction of thewindshield is generated at the outside in its peripheral portion (in adirection away from the driver). This is because the radius of curvatureat the peripheral portion becomes smaller than that at the center of thewindshield. In order to reduce this,

-   (1) The back surface reflected light is reduced by providing    antireflection coating on a surface of the windshield that is in    contact with the outside air.-   (2) Moreover, the inventors thought that a visual double image is    reduced in a projection optical system that generates a virtual    image by applying measures (will be described below) thereto.

Next, an embodiment in which the double image described above is to bereduced will be described in detail with reference to FIG. 10. Note thatin order to simplify explanation thereof, it will be described herein byusing a real image projection optical system.

FIG. 10 illustrates a relationship between an object point P1 and animage formation point P0 of the real image projection optical system. Asevaluation of the image forming performance, the image formingperformance on a panel surface is evaluated by evenly dividing anincident eye from a side of the virtual image (an original imageformation point) toward the panel surface (an original object point)that is an object point and causing a ray to fly.

The EyeBox of the driver is divided as illustrated in FIG. 11, and raytracking is carried out at equal intervals. Thus, the image formingperformance on the panel surface corresponding to each evaluation pointwas evaluated. In results based on lens data indicated in FIG. 14 andFIG. 15 according to the present embodiment, a size of aberrationoccurring at the center of the screen is different from that occurringat the peripheral portion as illustrated in FIG. 12 and FIG. 13. Forthis reason, a spot shape varies depending upon a position in thescreen, and spots does not become concentric with respect to a main raythat passes through the center of the incident eye (a point crossing theoptical axis), whereby blurring (or aberration) occurs.

Thus, with respect to design of the lens, in order to reduce aberrationthat occurs outside the main ray, it may be designed so that a raypassing through an upper peripheral portion of a pupil travel toward theinside of the main ray. Specifically, an optical element having a shape,by which refractive power by an optical path through which a ray passingthrough an upper peripheral portion than a ray position, through whichthe main ray passes, passes becomes relatively stronger compared withrefractive power by an optical path through which the main ray passes,may be arranged.

This will be described on the basis of a case of reverse tracking inwhich a ray is caused to fly from an image point toward an object pointas illustrated in FIG. 10. In this method, an object point is arrangedat a side of a virtual image; an image point is used as a panel surface;an incident eye of a virtual image optical system is evenly divided; anda ray is caused to fly, whereby image forming performance on a panelsurface is evaluated. In the virtual image optical system, relativerefractive power of the virtual image optical system of a meridional rayindicated by a solid line is also different from that of a sagittal rayindicated by a broken line.

For this reason, in a case where focus performance is set to best by thesagittal ray, the meridional ray focuses on in front of the panelsurface, and aberration occurs on the panel surface (which is displayedby gray (a meshed portion) in FIG. 10). For this reason, in a case wherethe information display apparatus 100 is arranged under the windshield,the double image generated by the back surface reflected light among thedouble images that may be generated on the windshield described above isgenerated at the upper portion of the video by the regular reflectedlight. Therefore, in order to reduce this, relative refractive power ofa virtual image optical system through which a ray passing through anupper portion than the center of an incident eye passes may be set to besmaller than relative refractive power of a virtual image optical systemthrough which a ray passing through the center and a lower portion thanthe center of the other incident eye passes so that aberration occursbelow the main ray.

On the other hand, the double image in the horizontal direction of thewindshield is generated at the outside in its peripheral portion (in adirection away from the driver). This is because the radius of curvatureat the peripheral portion becomes smaller than that at the center of thewindshield. For that reason, the double image generated by the backsurface reflected light is generated outside the video by the regularreflected light. Thus, in order to reduce this, relative refractivepower of a virtual image optical system through which a ray passingthrough the outside of the center of an incident eye passes may be setto be smaller than relative refractive power of a virtual image opticalsystem through which a ray passing through the center and an innerportion than the center of the other incident eye passes so thataberration occurs inside the main ray.

Further, in place of the design of the lens described above, asillustrated in FIG. 35, by curving a panel surface 4A, which is thevideo display apparatus 4 constituting the real image projection opticalsystem, so as to fit with a curved surface of the windshield, thesimilar effects described above can be obtained. More specifically, aradius of curvature in the vertical direction of the windshield issmaller than a radius of curvature in the horizontal direction. Thus, ina case where the windshield is replaced by a concave mirror, opticalpower in the vertical direction is large. For this reason, by making theradius of curvature in the vertical direction of the panel smaller thanthat in the horizontal direction and making the optical Petzval sumsmaller in the whole system, the curvature of field is reduced. This canbe realized by configuring the panel surface 4A to be curved in a convexmanner with respect to a light source 5A. Note that the windshield hasdifferent radii of curvature between the vertical direction and thehorizontal direction as described above. Therefore, it is preferablethat curvature of the panel surface 4A is also set appropriately inaccordance with the different radii of curvature of the windshield.

In order to take light in the virtual image optical system describedabove more effectively, as illustrated in FIG. 36, a radius of curvatureof a panel surface 4B may be matched up with a radius of curvature of alight source 5B.

Next, a configuration according to the embodiment of the presentinvention for allowing the double image described above to be reducedand allowing a virtual image with high visibility to be formed will bedescribed in detail with reference to FIG. 16 to FIG. 32. FIG. 16 is anenlarged view of a main portion of the liquid crystal panel and abacklight light source 5 as the video display apparatus 4 in the virtualimage optical system according to the embodiment described above. Bymodulating light from the backlight by a video signal inputted from aflexible board 10 of the liquid crystal panel, video is displayed on aliquid crystal panel display surface 11, and a virtual image isgenerated from the displayed video by the virtual image optical system(including a free-form surface concave mirror and a free-form surfaceoptical element in the embodiment) to inform the driver of videoinformation.

In the configuration described above, an LED light source withrelatively inexpensive and high reliability as a solid light source isused for a light source element of the backlight light source 5. Asurface emission type of LED is used for high output. Therefore, lightutilization efficiency is improved by using technical ingenuity (will bedescribed later). Emission efficiency of the LED with respect to inputelectric power is different depending upon emission color, and is about20% to 30%. The remaining is almost converted into heat. For thisreason, as a frame to attach the LED, by providing a fin 13 for heatradiation configured by a member with high thermal conductivity (forexample, a metallic member such as aluminum) to radiate heat to theoutside, an effect to improve the emission efficiency of the LED itselfcan be obtained.

In particular, emission efficiency of an LED using red as emissioncolor, which currently goes on the market, is significantly deterioratedwhen junction temperature becomes higher, and chromaticity of videochanges at the same time. Thus, it is preferable that the light sourceelement is configured so that priority to reduce temperature of the LEDis raised and an area of the corresponding radiation fin becomes largerto improve cooling efficiency. In order to guide light diffused from theLED to the liquid crystal panel 4 efficiently, a light guide element 18is used in the example shown in FIG. 17. However, it is preferable thatthe whole is covered by an exterior member 16 to unify the backlightlight source so that dust or the like does not adhere thereto, forexample.

Further, FIG. 17 shows an enlarged view of a main part of a light sourceunit including an LED that is a light source, a light guide element anda diffused plate. As is apparent from FIG. 17, by inserting a mediumbetween the LEDs to optically connect openings 21 a, 22 a, 23 a, 24 athat take in divergent light rays from LEDs of light funnels 21, 22, 23,and 24 to each other as a plane or by having a focusing action as aconvex shape to cause the light source light to be diverged to becomeparallel light as much as possible, an incident angle of light madeincident on a boundary surface of the light funnel is made smaller. As aresult, the divergent angle can further be made smaller after passingthrough the light funnel. Therefore, control of the light source lightdirected to the liquid crystal panel after being reflected by the lightguide element 18 can be facilitated.

Moreover, in order to improve utilization efficiency of divergent lightfrom the LED, polarization conversion is carried out for a joiningportion 25 between the light funnels 21 to 24 and the light guideelement 18 by using a PBS (Polarizing Beam Splitter) to convert that ofthe light to a desired polarization direction, whereby it is possible toimprove efficiency of incident light to the LCD.

As described above, in a case where the polarization direction of lightsource light is uniformed, it is further preferable that material withlow birefringence is used as raw material of the light guide element 18so that problems such as coloring at the time of black display does notoccur, for example, when a direction of the polarized wave is rotatedand the polarized wave passes through the liquid crystal panel.

As mentioned above, the light flux from the LED whose divergent angle isreduced is controlled by the light guide element; is reflected by atotally reflecting surface that is provided an inclined surface of thelight guide element 18; is diffused by a diffusing member 14 arrangedbetween an opposing surface and the liquid crystal panel; and is thenmade incident on the liquid crystal panel 4 as the video displayapparatus. In the present embodiment, as described above, the diffusingmember 14 is arranged between the light guide element 18 and the liquidcrystal panel 4. However, the similar effect can be obtained even thougha diffusing effect is caused to have an end surface of the light guideelement 18 to provide a fine uneven shape, for example.

Next, a configuration of the light guide element 18 described above andeffects obtained by the same will be described with reference to FIG.20. FIG. 20 is an outline view illustrating the light guide element 18according to the present embodiment. The light flux whose divergentangle is reduced by the light funnels 21 to 24 shown in FIG. 11 is madeincident on a light incident surface 18 a of the light guide element 18.At this time, the divergent angle of the vertical direction (verticaldirection in FIG. 21) is controlled by a shape (FIG. 21 shows across-sectional shape thereof) effect of the incident surface, and thelight flux efficiently transmits within the light guide element 18.

FIG. 21 is an enlarged cross-sectional view of a main portion of thelight guide element. The light source light whose divergent angle isreduced by the light funnels 21 to 24 is made incident from the incidentsurface 18 a via the joining portion 25 as described above, and istotally reflected by a prism 18 as the light guide element 18, which isprovided on the opposing surface, toward an opposing surface 17. A shapeof the totally reflecting prism 18 is divided and formed in a stepwiseshape in the vicinity (enlarged view of B portion) of and at an endportion (enlarged view of A portion) of the incident surface 18 a inaccordance with the divergent angle of the light flux made incident oneach surface, whereby an angle of the totally reflecting surface iscontrolled. On the other hand, an arrival position and an amount ofenergy of the divided light flux after reflection are controlled byusing a division dimension of the totally reflecting surface describedabove as variables so that light quantity distribution of the light fluxmade incident on the liquid crystal panel 4 that is the video displayapparatus becomes uniform in the emission surface of the liquid crystalpanel 4.

FIG. 18 shows a result obtained by simulating a state where emissionlight from the backlight described above passes through the liquidcrystal panel in the information display apparatus 100 according to thepresent embodiment. FIG. 18(a) is a view illustrating an emission stateof light when viewed from a longitudinal direction of the liquid crystalpanel. FIG. 18(b) is a view illustrating an emission state of light whenviewed from a short side direction (that is, a direction perpendicularto the longitudinal direction thereof) of the liquid crystal panel. Inthe present embodiment, the horizontal angle of the FOV is widened morethan its design. Therefore, it is designed that brightness of thevirtual image viewed and recognized by right and left eyes is notchanged extremely even in a case where a diffusion angle in a horizontaldirection is enlarged with respect to that in a vertical direction and aposition of eyes is moved by rotation of the driver's head.

Further, by making a divergent angle of the backlight in the verticaldirection smaller, a divergent angle of video displayed on the liquidcrystal panel in a vertical direction of the screen is made smaller,whereby generation of a double image is suppressed. FIG. 19 illustratesbrightness distribution of the emission surface of the liquid crystalpanel 4 in case of using the backlight by controlling an emissiondirection and intensity of light by using the light guide element 18like the present embodiment. As is apparent from FIG. 19, inclination ofdeterioration of brightness outside an effective range of the verticaldirection (long side direction) of the screen can be made smaller inaddition to the brightness distribution of the vertical direction (shortside direction) of the screen.

The emission light (video light) from the liquid crystal panel that isused as the video display apparatus in the information display apparatus100 according to the present embodiment indicates predeterminedtransmittance in a range of ±50° as shown in FIG. 23 and FIG. 28 in acase where visual angles in horizontal and vertical directions are usedas a parameter (see FIG. 22). In a case where a range of the visualangle falls within ±40°, it is possible to obtain abetter transmittancecharacteristic. As a result, as shown in FIG. 24 and FIG. 29, brightnessof the screen greatly varies depending upon a direction to view thescreen (visual angle) in the horizontal direction and the verticaldirection of a display screen. This is caused by an angularcharacteristic of backlight brightness shown in FIG. 25 and FIG. 26.

For this reason, the inventors obtained high brightness by controllingthe angle of the totally reflecting surface of the light guide element18 and the divergent angle of the light source light from the LEDs ofthe light funnels 21 to 24 to narrow visual angle property of thebacklight into a small range so that the emission light from the liquidcrystal panel 4, which is taken in the virtual image optical system, canbe obtained as light vertical to the screen as much as possible.Specifically, as shown in FIG. 24 and FIG. 29, in order to obtain videowith high brightness, light in a range of ±30° in a right-and-leftviewing angle is used, and contrast performance shown in FIG. 27 andFIG. 31 is considered. By narrowing it to ±20° or smaller, the virtualimage using a source image with good image quality could be obtained atthe same time.

As mentioned above, the contrast performance that influences the imagequality of the video display apparatus is determined by to what extentbrightness when black as a basis to determine image quality is displayed(in FIG. 30 and FIG. 32, indicated by “black display brightness”) can belowered. For this reason, it is preferable that an iodine-basedpolarizing plate in which the degree of polarization is high is usedbetween the liquid crystal panel 4 and the backlight.

On the other hand, by using a dye-based polarizing plate as a polarizingplate provided at the optical element 2 side (light emission surface),it is possible to obtain high reliability even in a case where outsidelight enters therein or ambient temperature is high.

In a case where color display is executed in the liquid crystal panel 4,color filters corresponding to the respective pixels are provided. Forthis reason, in a case where light source color of the backlight iswhite, light absorption with the color filters becomes greater, and lossthereof becomes larger. Thus, as shown in FIG. 17 described above, theinventors use a plurality of LEDs:

-   (1) to add a green LED whose contribution to brightness is large    compared with the case where a plurality of white LEDs is used.-   (2) to add a red or blue LED to the white LED to improve glossy    property of an image.-   (3) to individually arrange red, blue, and green LEDs, add a green    LED whose contribution to brightness is large, and individually    drive the LEDs, thereby enlarging a color reproduction range to    heighten glossy property as well as to improve brightness.-   (4) to raise transmittance of each color filter with respect to peak    brightness of red, blue, green LEDs by implementing the above (3) to    improve brightness as a whole.-   (5) to reduce damage of the polarizing plate at an incident side of    the liquid crystal panel as a second embodiment of the backlight by    arranging the PBS between the light funnels and the light guide    element to uniform it to a specific polarized wave.    It goes without saying that a polarization direction of the    polarizing plate that is arranged at the incident side of the liquid    crystal panel may be a direction through which the polarized wave    uniformed to a specific direction after passing through the PBS    (Polarizing Beam Splitter) passes.

As mentioned above, as the video display apparatus 4 according to theembodiment of the present invention, a λ/4 plate can be provided on theemission surface of the liquid-crystal display panel to convert theemission light to circular polarized light. As a result, the driver isallowed to monitor a good virtual image even though he or she wearspolarized sunglasses.

Moreover, by forming reflection coating for the reflecting mirror usedin the virtual image optical system by metallic multilayer, angulardependence of reflection power is small, and the reflection power isnever changed by a polarization direction (a P wave or S wave).Therefore, it becomes possible to uniformly keep chromaticity andbrightness of the screen.

Moreover, in a case where an ultraviolet ray reflecting film or anoptical member obtained by combining the ultraviolet ray reflecting filmand an infrared ray reflecting film is provided between the virtualimage optical system and the windshield, temperature rise of theliquid-crystal display panel and the polarizing plate and damagetherefrom can be reduced even though outside light (solar light) entersit. Therefore, an effect that reliability of the information displayapparatus 100 is not impaired can be obtained.

Further, in the virtual image optical system, optimum design including adifference between the radius of curvature in the horizontal directionand the radius of curvature in the vertical direction of the windshieldthat is the projected member in the conventional technique in thevehicle is carried out. The concave mirror 1 whose concave surface facesthe windshield 6 side is arranged between the windshield 6 and the videodisplay apparatus 4 or an intermediate image display unit. This causesvideo of the video display apparatus 4 to be enlarged, and the video isreflected by the windshield 6. At this time, the optical element isarranged between the concave mirror 1 described above and the videodisplay apparatus 4. On the other hand, video light flux to form anenlarged image (virtual image) of the video, which is formed so as tocorrespond to a viewpoint position of the driver, passes through theoptical element arranged between the concave mirror 1 and the videodisplay apparatus 4, thereby correcting distortion and/or aberrationthat occurs in the concave mirror 1. For that reason, a virtual image inwhich distortion and aberration are reduced significantly can beobtained compared with the virtual image optical system including onlythe conventional concave mirror.

Moreover, in the configuration according to the present embodiment shownin FIG. 1, it is necessary to form a virtual image obtained by beingreflected at the upper portion of the windshield 6 (that is, an upperportion in a vertical direction of the vehicle body) at a far position.For this reason, in order to favorably form the video light fluxdiverged from the upper portion of the video display apparatus by whichthe video corresponding to this is displayed, it is necessary that afocal point distance f1 of the optical element arranged between theconcave mirror 1 described above and the video display apparatus 4 ismade shorter, and to the contrary, the virtual image obtained by beingreflected at the lower portion of the windshield 6 (that is, a lowerportion in a vertical direction of the vehicle body) is formed in thevicinity thereof. For this reason, in order to favorably form the videolight flux diverged from the lower portion of the video displayapparatus by which the video corresponding to this is displayed, acomposite focal point distance f2 of a plurality of optical elementsarranged between the concave mirror 1 described above and the videodisplay apparatus 4 may be set to be relatively longer.

Further, in the present embodiment, the radius of curvature in thehorizontal direction (parallel to the ground) of the windshield 6 isdifferent from the radius of curvature in the vertical direction (thatis, a direction vertical to the horizontal direction of the windshield),whereby screen distortion of the virtual image viewed by the driver iscorrected. Therefore, by arranging the optical element whose axissymmetric property is different with respect to the optical axis in thevirtual image optical system, correction of the distortion describedabove is achieved.

As described above, the sheet-like light source apparatus suitable to beused for an electronic apparatus provided with the image display deviceaccording to various embodiments of the present invention has beendescribed. However, the present invention is not limited to theembodiments described above, and various modifications are contained.For example, the whole system has been explained in detail in theembodiments described above for explaining the present inventionclearly. The present invention is not necessarily limited to one thatincludes all configurations that have been explained. Further, a part ofthe configuration of one embodiment can be replaced by a configurationof the other embodiment. Further, a configuration of the otherembodiment can be added to a configuration of one embodiment. Moreover,a part of the configuration of each of the embodiments can be added tothe other configuration, deleted or replaced thereby.

REFERENCE SINGS LIST

100 . . . information display apparatus, 101 . . . vehicle, 1 . . .concave mirror, 2 . . . optical element, 4 . . . video displayapparatus, 4A, 4B . . . liquid-crystal display panel, 5A, 5B . . .backlight light source, 6 . . . projected member (windshield), 7 . . .housing, V1 . . . virtual image, 8 . . . EyeBox (eyes of observer), 9 .. . light source unit, R1 . . . upper video light, R2 . . . centralvideo light, R3 . . . lower video light, 10 . . . flexible board, 11 . .. video display surface, 12 . . . frame, 13 . . . fin, 14 . . .diffusing member, 16 . . . exterior member, 17 . . . emission surface,18 . . . light guide element, 20 . . . light funnel unit, 21 to 24 . . .light funnel, 36 . . . emitted ray from liquid crystal panel.

The invention claimed is:
 1. An information display apparatus configuredto display video information of a virtual image on a reflecting surfaceof conveyance, the information display apparatus comprising: a displayconfigured to display the video information; and a virtual image opticalsystem configured to display a virtual image at a front of theconveyance by reflecting light emitted from the display by means of thereflecting surface, wherein the virtual image optical system includes aconcave mirror and an optical element, wherein the optical element isarranged between the display and the concave mirror, wherein a shape ofthe optical element is configured so as to correct distortion aberrationand aberration, the distortion aberration occurring due to an opticalpath difference generated by a difference between a radius of curvatureRv in a vertical direction and a radius of curvature Rh in a horizontaldirection of the reflecting surface by setting an average radius ofcurvature of a cross-sectional shape in the vertical direction and anaverage radius of curvature of a cross-sectional shape in a horizontaldirection to different values, the aberration deteriorating imagingperformance of the virtual image, wherein in the virtual image opticalsystem: refractive power by an optical path through which a light fluxpassing through an outside of a main ray among a video light flux passesis set to be stronger compared with refractive power by an optical paththrough which the main ray passes, the video light flux converging in animage point of a virtual image surface by the video light flux fromrespective object points of video light from the display; the reflectingsurface has a shape in which the radius of curvature becomes smaller ata peripheral portion compared with a center of an outer shape of thereflecting surface; and a video light ray is separated into regularreflected light reflected by a first reflecting surface of thereflecting surface positioned at an internal side of the conveyance andback surface reflected light reflected by a second reflecting surfacethat is in contact with an outside air, and in a double image generatedby the regular reflected light and the back surface reflected light, adirection in which the double image is generated is different between avertical direction and a horizontal direction of the reflecting surface,and wherein to reduce a double image that is generated in the horizontaldirection of the double image, with respect to the virtual image that isformed at a peripheral portion of the reflecting surface, relativerefractive power of a virtual image optical system through which a raypassing through an outside of a center of an incident eye of the virtualimage optical system passes is set to be smaller than relativerefractive power of a virtual image optical system through which a raypassing through an inner portion than a center of the other incident eyepasses so that aberration occurs below the main ray of the virtual imageoptical system.
 2. The information display apparatus according to claim1, wherein the virtual image optical system includes an optical elementwith weak negative refractive power, and the optical element with weaknegative refractive power is configured to emit, after video light fromthe display is focused on and made incident on the optical element tobecome substantially parallel light, the substantially parallel lighttoward the concave mirror.
 3. The information display apparatusaccording to claim 1, wherein the virtual image optical system includesan optical element with weak positive refractive power, and the opticalelement with weak positive refractive power is configured to emit, aftervideo light from the display is diverged and made incident on theoptical element to become substantially parallel light, thesubstantially parallel light toward the concave mirror.
 4. Theinformation display apparatus according to claim 1, wherein the displayis arranged so that a center of a video display unit thereof in avertical direction of a screen is positioned at a lower side than acenter of the concave mirror.
 5. The information display apparatusaccording to claim 1, wherein a liquid-crystal display panel is used asthe display.
 6. The information display apparatus according to claim 5,wherein the liquid-crystal display panel that is the display includes: asolid light source; an optical element configured to cause a divergentangle of divergent light from the solid light source to be smaller; anoptical member configured to subject the divergent light whose divergentangle is caused to be smaller by the optical element to polarizationconversion; and an illuminating light source unit configured to causelight with a desired polarization direction to be incident on theliquid-crystal display panel.
 7. The information display apparatusaccording to claim 6, wherein after emission light from the illuminatinglight source unit passes through the liquid-crystal display panel, anemission state of light when viewed from a longitudinal direction of theliquid-crystal display panel is different from an emission state oflight when viewed from a short direction thereof.
 8. The informationdisplay apparatus according to claim 5, further comprising: a λ/4 plateprovided at a side of the virtual image optical system of theliquid-crystal display panel that is the display.
 9. The informationdisplay apparatus according to claim 5, further comprising: an opticalmember configured to reflect any one of an ultraviolet ray, an infraredray, or both the ultraviolet ray and the infrared ray, the opticalmember being provided at a side of the virtual image optical system ofthe liquid-crystal display panel that is the display.